A printed circuit board is a structural element formed of insulated materials together with conductor wirings, and is applied to various communication and electronic devices. Printed Circuit Board technologies have been developing continuously, enabling an initial single sided board to evolve into a double sided board and further into a multi-layer board. At present, the four-layer board and six-layer board are dominant in wide applications. The development trend of the printed circuit board is High Density Interconnection (HDI) technology, in which high density interconnection is achieved mainly by arranging microvias together with fine lines with the use of microvias technology, so that a space utilization ratio is improved. Typically, in the microvias technology, Laser Ablation, Plasma Etching or Photo-via can be used.
At present, a structure of a six-layer HDI broad, including an HDI board of a six-layer one-level 1+4+1 structure or an HDI board of a six-layer two-level 1+1+2+1+1 structure, is typically adopted for a terminal-like mainboard PCB. As the integration level of devices is improved increasingly, a hardware cost of the PCB is lowered continuously. The cost of the mainboard PCB is an increasing proportion of the hardware cost of a whole machine, and the signal quality in the PCB also has a direct influence on the performance of the whole device. Therefore, the mainboard PCB is a major factor having an influence on the performance and the price of a terminal product.
However, the existing six-layer HDI board has a relatively high cost due to excessive fabrication processes, a long process cycle and numerous board material layers. Further, the six-layer HDI board can not be easily made thin in shape due to the fact that the insulation strength between every two of the numerous layers shall be guaranteed necessarily, and that a dielectric layer between respective signal layers shall not be too thin because a too thin inner dielectric layer may result in a lowered qualified rate directly. Currently, a non-laser via dielectric layer by most PCB manufacturers typically has a dimension of above 4 mil due to limitations of the technologic level, and therefore the existing six-layer one-level HDI board has a typical thickness of 0.8 mm or above, and the fabrication cost will be increased in proportion to a thickness below 0.8 mm.